Abstracts

Cortical stimulation is playing an increasing role in clinical neuroscience in the treatment of epilepsy, stroke and pain. The neuromodulatory effects of cortical stimulation include both excitation and inhibition, the balance of which varies with the stimulation parameters. As a result, cortical stimulation also influences cerebral hemodynamics, including perfusion and oxygenation, which may also vary depending on the stimulation parameters, although little data exists on this subject. In this study, we investigate the effects of stimulation frequency on the hemodynamic response using optical recording of intrinsic signals (ORIS)., Using bipolar tungsten electrodes, the rat sensory neocortex was stimulated at variable frequencies in a pseudo-random fashion at 5 Hz, 10 Hz, 50 Hz, 100 Hz, and 250 Hz at a constant charge (CC). The amplitude remained constant at 1 mA. CC was maintained by delivering stimulation at 100 spikes at each frequency and varying the duration of stimulation. Simultaneous field potential (f.p.) recording and ORIS was performed at 570 nm and 610 nm to quantitatively measure total hemoglobin (Hbt), deoxygenated hemoglobin (Hbr) and oxygenated hemoglobin (HbO2) using a pathlength correction. Optical data was averaged over 10 trials at each frequency and each trial lasted 100 sec so the optical signal could return to baseline. Trials with afterdischarges were eliminated., A large increase in Hbr was recorded at all stimulation frequencies but the maximum occurred at 10 Hz (2.12[plusmn]0.20 % , n=3). The relationship between the increase in Hbr and stimulation frequency (in log scale) could be characterized by the equation y = -1.1164x2 + 3.1836x - 0.2979 (R2=0.74). There was a significant correlation between the duration of stimulation and the duration of the increase in Hbr (r=.86,p[lt]0.05). The largest increase in Hbt occurred at 50 Hz (4.04[plusmn]1.08 % , n=3) and the relationship between the increase in Hbt and stimulation frequency (in log scale) could be characterized by the equation y = -1.3816x2 + 5.3414x - 1.2291 (R2=0.98, p[lt]0.05). The duration of the increase in Hbt was generally longer than the duration of stimulation by 80 seconds, regardless of frequency., These results demonstrate that the duration of stimulation is more important than the frequency in determining the duration of the dip and that the relationship between frequency and both Hbt and Hbr is not linear, with a plateau occurring between 10-50 Hz., (Supported by NIH.)